A highly efficient procedure for transferring functional genes into mammalian cells has been developed using retroviral-based vectors as a delivery system. When mouse bone marrow cells are infected in vitro with a neo-R gene and reinjected into a lethally irradiated recipient mouse, 90% of the stem cells (CFU-S) can be shown to carry an intact copy of the neo-R gene. The majority of these cells produce the neo-R gene product: phosphotransferase. Retroviral vectors containing the human gene for the enzyme adenosine-deaminase (ADA) have been made and inserted into mice, monkeys, and ADA-human T and B cells. These studies are preliminary to attempting human gene therapy in patients suffering from ADA - severe combined immunodeficiency disease.